Attempts have been made to develop purification methods whereby the COD of a photographic waste solution can be. effectively reduced, those whereby nitrogen, phosphorus, etc. contained in a waste solution can be eliminated, those whereby heavy metals contained in a waste solution can be eliminated and those whereby advantages can be achieved at the same time. For example, there have been carried out or proposed biological treatments, burning, evaporation to dryness by heating, oxidation with the use of chlorine-based chemicals, electrolytic oxidation, hydrogen peroxide/ferrous salt treatments, ozone oxidation, coagulation with the use of inorganic or organic flocculating agents, adsorption with the use of activated carbon, inorganic adsorbents or organic polymers, reverse osmosis with the use of membranes, electric dialysis and ultrafiltration.
However, each of the above-mentioned methods requires possible improvement due to the following problems. That is to say, (1) none of the biological treatments, coagulation, adsorption, electric dialysis, reverse osmosis, ultrafiltration, hydrogen peroxide/ferrous salt treatments and ozone oxidation can establish the purification at a satisfactory level, when employed alone; (2) each of the treatments such as burning, evaporation to dryness, electrolytic oxidation, ozone oxidation, adsorption, reverse osmosis, electric dialysis and ultrafiltration, requires a high treatment cost; and (3) each of the treatments such as burning, evaporation to dryness, coagulation and adsorption suffers from a problem of the formation of substances which might cause secondary pollution after the completion of the treatment.
Under these circumstances, the present invention has been completed. Accordingly, the present invention is to provide a process for purifying a photographic waste solution which makes it possible to lower the COD of the photographic waste solution to a satisfactorily low level and to eliminate nitrogen, phosphorus, heavy metals, etc. therefrom at a low cost without forming any substance causing secondary pollution or requiring troublesome procedures for the maintenance.
To solve the above-mentioned problems, the present invention provides a process for treating a photographic waste solution, which can be easily carried out and is excellent in practicality and economy, which comprises subjecting the photographic waste solution to noncatalytic wet oxidation at a temperature of from 140.degree. C. up to 370.degree. C. under such a pressure as to hold said waste solution in a liquid phase while supplying an oxygen-containing gas thereto; eliminating the solid matters and/or suspended matters thus formed; effecting catalytic wet oxidation at a temperature of from 140.degree. C. up to 370.degree. C. under such a pressure as to hold said waste solution in a liquid phase while supplying an oxygen-containing gas thereto; and further subjecting the treated solution after the catalytic wet oxidation to a biological treatment.
A hardly degradable photographic waste solution is difficult to make completely harmless within a practically available period of time and at a reasonable cost, even though noncatalytic wet oxidation is combined with catalytic wet oxidation. Moreover, there arises a problem of changes in the components. Thus the present inventors have employed a combination of a biological treatment, which is scarcely affected by the changes in the components, with the above-mentioned treatments and thus successfully provided a process for treating a photographic waste solution which is excellent in practicality and economy.